Boron nitride film capacitor



Dec. 19, 1967 R PATTERSON ET AL 3,359,468

BORON NITRIDE FILM CAPACITOR 4 Sheets-Sheet 1 Original Filed Dec. 12, 63

- METAL SUBSTRATE B- frichloroborazole FIG. I

VACUUM PUMP FIG. 8

TIME- HOURS BORON NITRIDE FILM METAL BORIDE INTERLAYER METAL SUBSTRATEINVENTOR5 mf Z M W Z M. WM 0; W fi w Dec. 19, 1967 R PATTERSON ET AL3,359,468

BORON NITRIDE FILM CAPACITOR Original Filed Dec. 12, 1963 4 Sheets-Sheet2 46' 44\UJ 7 Aluminum Film Electrode 42 \\\\H\\\\\\\\\\\\\\\\\\ \\\w nMolybdenum Boride lnterloyer 0 Molybdenum Foil Substrate bQeQ O Q Q Fi.4

Gold Film Electrode I00 4 Boron Nitride Film 9 Molybdenum Boridelnterlayer Molybdenum Foil Substrate Molybdenum Boride lnterlayer g;Boron Nitride Film [02 Gold Film Electrode 158 1 Molybdenum FilmElectrode Boron Nitride Film Molybdenum Borlde lnterldyer MolybdenumFilm Electrode Boron Nitride Film Molybdenum Boride lnterlayerMolybdenum Film Electrode Boron Nitride Film Molybdenum Boridelnterloyer Molybdenum Foil Substote Fly. 10

iNVENTORg Dec. 19, 1967 R PATTERSON 'ET AL 3,359,468

BORON NITRIDE FILM CAPACITOR Original Filed Dec. 12. 1963 4 Sheets-Sheet4 W )2 m offformeg United States Patent G 10 Claims. Cl. 317-258) Thisapplication is a division of application Ser. No. 330,023, filed Dec.12, 1963.

The present invention relates to electrical capacitors, and moreparticularly but not by way of limitation, relates to an improvedcapacitor having a thin boron nitride film as a dielectric.

It is well-known in the art that capacitance is directly proportional tothe area of the opposed electrodes and inversely proportional to thethickness of the dielectric between the electrodes. The voltage at whichthe capacitor may be operated is determined by the breakdown voltage ofthe dielectric. Since it is desirable to have capacitors of minimumsize, but maximum capacitance and maximum breakdown voltage, attemptshave been made to produce very thin dielectrics for use in capacitorshaving high insulation values.

Due to the outstanding dielectric properties of boron nitride, attemptshave been made to construct capacitors using pure boron nitride as thedielectric. Substantially all of these attempts have involved what mightbe termed the bulk approach in that the dielectric layer is constructedby working a mass of boron nitride down to a thin sheet. For example,solid boron nitride has been successfully machined down to a thicknesson the order of 25 mils and then placed between suitable electrodes suchas metal films painted on the opposite surfaces of the thin sheet. Or,powdered boron nitride has been combined with suitable binding materialand subjected to high pressure and high temperature toform a relativelythin pressed dielectric sheet.

In the first instance, the minimum thickness of the machined boronnitride is limited by the machining process, except when using verycostly machining techniques, to a minimum thickness of about 25 mils ascompared with an optimum thickness of substantially less than one mil.In the second case, the intermixed binders and other additivesappreciably decrease the insulation properties of the boron nitride.Further, the binders produce small zones in which no boron nitride ispresent. Therefore, as the sheet is pressed thinner, some areas betweenthe electrodes do not have any boron nitride present and the capacitorshorts out or has a low breakdown voltage.

Other attempts to construct very thin film dielectrics for use incapacitorshave entailed the production ofa metal oxide film or othermetal compound film on a metal substrate. While these capacitors aresuitable for some purposes, metal ions must diffuse into the metal oxidedielectric being formed in order to carry out the oxidation reaction.This results in a metal ion gradient throughout the film whichappreciably reduces the dielectric quality of the film.

More recently, it has been suggested that boron nitride can be depositeddirectly upon a metal substrate by the thermal decomposition oftrichloroborazole and that the boron nitride deposited in this mannercould 'be used for electrical capacitors. However, it has been foundthat boron nitride cannot be deposited directly upon a metal substrateto produce a thin, continuously dense film which is strongly adherent tothe metal substrate so as to be used to manufacture electrical devices,and in particular, the quality and thickness of the film could not besufiiciently controlled for commercial application.

In accordance with the broader aspects of the present invention, acapacitor is constructed by adherently depositing a film of boronnitride from the vapor phase onto a metallic substrate. The film is lessthan two mils in thickness. The boron nitride film is bonded to thesubstrate by an interlayer comprised of a compound of the substratematerial, and preferably is either a boride or a nitride. Then ametallic film counterelectrode is deposited on the boron nitride filmwhich serves as the dielectric. The boron nitride film is very thin,pure, dense, and strongly adherent to the metallic substrate such thatvery small capacitors having very high capacitance values can beconstructed.

-In accordance with another aspect of the invention, a plurality ofboron nitride films of different thicknesses are deposited betweenmetallic film electrodes to produce a highly versatile capacitor.

As mentioned, very thin boron nitride films permit the construction ofextremely small capacitors having high capacitance values. However,connecting electrical leads to the appropriate electrodes constitutes amajor problem. Therefore, the present invention further contemplatesseveral novel processes using selective etching, masking and abrasiveetching for making the necessary electrical connections to theelectrodes of capacitors, and in particular to the electrodes ofmultilayer capacitors.

Specifically, the present invention contemplates that the electricalconnections can be made after using selective etching techniques on theboron nitride and the metallic substrate and electrodes. In thisconnection, boron nitride has heretofore been considered assubstantially stable in all acids and bases and mixtures thereof.However, we have discovered that boron nitride can be etched by hotphosphoric acid, such that a novel selective etching process can beemployed as hereafter described in greater detail.

The invention also contemplates a micro-circuit comprised of a resistorand capacitor connected in series and comprised of a thin metal film, athin boron nitride dielectric film and a counterelectrode. The thinmetal film can be tantalum and serves as both a resistor and-anelectrode for the capacitor.

Therefore, an object of the present invention is to provide an improvedminiaturized capacitor having outstanding operating characteristics.

A still further object of this invention is to provide a miniaturizedmultilayer capacitor having a large number of selectable capacitances.

Another object of this invention is to provide a miniaturized capacitorhaving a high capacitance and a high breakdown voltage.

Yet another object of this invention is to provide a capacitor which isvery stable over a wide temperature range and over extended periods oftime.

Many additional objects and advantages will be evident to those skilledin the art from the following detailed description and drawings,wherein:

FIGURE 1 is a schematic drawing of an apparatus which may be used topractice the method of the present invention;

FIGURE 2 is an enlarged, schematic, partial sectional view of an articleof manufacture constructed in accordance with this invention;

FIGURE 3 is an enlarged, partial sectional view of a capacitorconstructed in accordance with the present invention;

FIG. 4 is a somewhat schematic plan view which serves with the presentinvention;

FIGURE 7 is a graph illustrating still another operating characteristicof five capacitors constructed in accordance with the present invention;

FIGURE 8 is a graph illustrating yet another operating characteristic ofthe five capacitors constructed in accordance with the presentinvention;

FIGURE 9 is an enlarged partial sectional view of still anothercapacitor constructed in accordance with the present invention;

FIGURE 10 is an enlarged partial sectional view of still anothercapacitor constructed in accordance with the present invention;

FIGURE 11 is a broken, enlarged sectional view illustrating the mannerin which electrical connections can be made to the electrodes of thecapacitor shown in FIG- URE 9;

FIGURE 12 is a broken, enlarged sectional view of another manner inwhich electrical connections can be made to the electrodes of thecapacitor shown in FIGURE 9; and,

FIGURE 13 is an enlarged, somewhat schematic sectional view of stillanother capacitor constructed in accordance with the present invention.

In accordance with the present invention, an interlayer is used tochemically bond a thin film of boron nitride to a metal substrate. Theinterlayer is formed by reacting a reagent with the metal substrate toform a metal boride, metal nitride, metal oxide or other interlayerwhich will chemically bond with a boron nitride film subsequentlydeposited on the interlayer. It is believed that such an interlayer can.be applied to substantially all refractory metals which will withstandthe temperatures used in the process, and is specifically applicable tomolybdenum, niobium, tantalum, tungsten and to alloys comprisedprimarily of these metals, and to certain ferrous base alloys.

In a specific embodiment of the invention, boron is reacted with themetal substrate to produce a metal boride interlayer. The boron may besupplied by reacting boron trichloride vapor with the surface of aheated substrate.

The boron nitride film may be formed by the reaction of boron andnitrogen from any suitable source reagents or a heated substrate uponwhich an interlayer has been formed. In a specific embodiment, howeverthe boron nitride film is deposited by the thermal decomposition ofbeta-trichloroborazole on a heated substrate. Or the boron and nitrogenmay be supplied by boron trichloride and ammonia.

The formation of the interlayer and the deposition of the boron nitridefilm are preferably accomplished in separate steps so that a uniforminterlayer is assured over the entire surface to be coated. A uniformboron nitride layer can then be deposited and the thickness of the filmrather precisely controlled. However, the two steps can be accomplishedsimultaneously by supplying a mixture of boron trichloride andtrichloroborazole vapors at the surface of a heatedsubstrate. r

A suitable apparatus for performing the process of the present inventionis indicated generally by the reference numeral 10 in FIGURE 1. Theapparatus 10 comprises a reaction chamber 12 formed by a bell jar 14 anda base 16. The reaction chamber 12 can be evacuated by a suitable pump18. A valve 20 is provided for sealing the reaction chamber from thepump when desired. A metal or other substrate 22 upon which the boronnitride film is to be deposited is held by suitable clamps 24 throughwhich an electrical current can be passed to heat the substrate. A valve26 controls theintroduction of boron trichloride vapors into thereaction chamber 12 from a suitable boron trichloride source 28. Asimilar valve 30 controls the introduction of trichloroborazole vaporsto the reaction chamber 12 from a trichloroborazole source 32 which mayconveniently comprise a vacuum container in which trichloroborazolecrystals can be heated and vaporized. In the event boron trichloride andammonia are used for the deposition of the boron nitride film, the valve30 can be used to control the introduction of the ammonia or the vaporsof the addition compound ammonia-boron trichloride which has beenpreviously formed by a reaction between boron trichloride and ammonia tothe reaction chamber 12.

When using the apparatus 10 to produce the capacitor of the presentinvention, the metal substrate 22 is preferably degreased by a suitablesolvent prior to placement in the reaction chamber 12. Next the reactionchamber 12 is evacuated by the pump 18 to a very low pressure,preferably less than about one micron of mercury. The substrate 22 maythen be heated in the vacuum to sublimate all volatile impurities whichare purged from the system by the vacuum pump 18. Next the reactionchamber 12 is backfilled with boron trichloride from the source 28 tothe desired pressure, which is preferably between about 5 and about 100microns of mercury. The substrate 22 is then resistively heated to atemperature in the range from about 900 C. to about 1300 C. for a shortperiod of time, usually from about 15 to about 60 seconds. During thisperiod the metal boride interlayer is formed upon the surface of themetal substrate 22 by the interaction of the metal and the bOIOn fromthe boron trichloride vapors.

The reaction chamber 12 is then evacuated to a low pressure of about onemicron of mercury in order to remove the remaining boron trichloridevapors and the freed chlorine. The reaction chamber 12 is thenbackfilled with beta-trichloroborazole vapors to a pressure betweenabout 10 and about 300 microns of mercury. Next the substrate 22 isheated to a temperature between about 900 C. and about 1300 C. until afilm of boron nitride of the desired thickness is achieved. A periodfrom about 1 to about 5 minutes is normal. The thickness of the film canbe rather precisely determined by controlling the various parameters ofpressure, temperature and contact time between the vapors and thesubstrate. Boron nitride films from 0.2 micron to 5 microns have beenprepared. The film can be made so thin and of such uniform thickness asto be substantially one color if the substrate is uniformly heated.

The boron nitride films are chemically stable in a wide variety ofcorrosive media. Specially, the boron nitride films are insoluble in HO, HCl (dilute and concentrate), HNO (dilute and concentrate), H (diluteand concentrate), aqua regia, NaOH, KOH (dilute and concentrate),cryolite mixtures, and HF (dilute and concentrate). The boron nitridefilms are stable in air up to 700 C., but are unstable above 800 C.after 30 minutes.

However, we have discovered that the boron nitride films are subject torapid attack by H PO at about 295 C. This unique feature permitsselective etching of the boron nitride to facilitate the construction ofthe capacitor as will hereafter be described in greater detail. In aspecific example, a strip of molybdenum coated with a boron nitride filmdeposited in accordance with the present invention and a second weighedstrip of molybdenum were placed in a solution of H PO that had beenboiled until bubbles had quit coming up and which was at approximately295 C. The strips were held in the H PO solution about five minutesbefore being removed and washed. The boron nitride film was removed fromthe molybdenum strip and electrical conductance by point contact to thestrip was good. The other weighed strip of molybdenum experienced noweight loss, indicating that molybdenum is stable is the hot H PO Anovel and highly useful electrical capacitor is manufactured bydepositing a very thin boron nitride film on a metal substrate using theinterlayer to chemically bond the film to the substrate as abovedescribed, and then depositing a metallic counterelectrode on thedielectric boron nitride film.

In order to test the continuity and uniformity of a boron nitride filmand to construct a capacitor, a molybdenum foil substrate 40 (FIGURE 3)was placed in the reaction chamber 12. A molybdenum boride interlayer 42and a boron nitride film 44 were then deposited on the molybdenumsubstrate using the process described above. A series of randomly-placedaluminum electrodes 46 of progressively increasing diameters asillustrated in FIG- URE 4 were then vacuum-evaporated onto the surfaceof the boron nitride film using conventional techniques. The aluminumfilm electrodes 46 had successively increasing areas up to 1.208 squareinches. The breakdown voltage of the boron nitride film between each ofthe electrodes 46 and the metal substrate was uniformly about 500 voltsDC. This indicates that the boron nitride is very uniform andcontinuously dense over the entire substrate because any defect in theboron nitride film under any electrode would result in a decrease in thebreakdown voltage.

Further tests of capacitors constructed in accordance with the presentinvention indicated that a 0.5 micron thick boron nitride film has adielectric strength of 5000 DC volts per mil (2 10 DC volts percentimeter) and a dielectric constant of 4.4. Thus a boron nitride filmone micron thick can be expected to have a breakdown voltage of 200 DCvolts and a capacitance of 25,000 micromicrofarads per square inch. TheDC voltage breakdown tests referred to above correspond to Method 301 ofthe procedures outlined in Mil-std-202 and represent the ability of theboron nitride film to withstand an impressed DC voltage over a period inexcess of one minute.

The variations in breakdown voltage for three separate capacitorsconstructed in accordance with the present invention are represented bythe curves 50, 52 and 54 on the graph of FIGURE 5. The boron nitridefilm of the capacitor represented by the curve 52 was thicker than theboron nitride coating of the capacitor represented by the curve 50, andthe boron nitride film of the capacitor represented by the curve 54 wasstill thicker. The voltage measurements were made in a dry nitrogenambient by making a point contact to the aluminum film electrodedeposited on the boron nitride film. Considering some inaccuracies inthe breakdown voltage measurements, it will be evident that no largechanges in the rate of degradation of the breakdown voltages withtemperature occurs with decreasing film thicknesses up to about 150 C.where the breakdown voltage of the capacitor having the thinnest boronnitride film represented by curve 50 begins to decrease at agreaterrate. In other words, the slopes of the curves 50, 52 and 54 aresubstantially the same constant value except for the curve 50 above 150'C. Considering some inaccuracies in the test measurement, this indicatesthat the rate of degradation for the thinnest boron nitride film mightincrease more rapidly above 150 C., whereas no change is apparent in thethicker boron nitride films represented by the curves 52 and 54.

Referring now to FIGURE 6, the curve 60 represents the percent change incapacitance with respect to temperature of five randomly-selectedcapacitor assemblies selected from a group constructed by encapsulatinga sandwich of molybdenum substrate, metal boride interlayer, boronnitride film and aluminum film electrode in transistor cans or in glassin conventional manner. The assemblies rather uniformly had capacitancevalues of 320 micromicrofarads and 200 volts breakdown. The temperaturecoefficients calculated from the curve 60 are 25 p.p.m./ C. at 55 0.,+17 p.p.m./ C. at 85 C., +10 p.p.m./ C. at 125 C., and +16 p.p.m./ C. at250 C. The overall accuracy of the capacitance measurements areapproximately 10.2 micromicrofarads or iS-lf) p.p.m./

C. No change in capacitance was detectable between 200 and 250 C. Fromthe above data, it will be evident to those skilled in the art thatcapacitors constructed in accordance with the present invention have anexceptional temperature stability.

The dissipation factors of a random group of 13 capacitor assemblieswere measured to be in the range of 0.01% at 1 kc. The boron nitridecapacitors which were carefully encapsulated in glass envelopes did notexhibit any change in the dissipation factor with temperature in therange measured, which was up to 175 C. Other capacitors encapsulated intransistor cans exhibited a slight increase in the dissipation factor,but even these compared very favorably with electrostatic precisionunits presently on the market.

Five capacitors constructed in accordance with the present inventionwere tested over extended periods of time to determine the percentchange in capacitance of each and the percent change in the dissipationfactor of each. The changes in the capacitance of each of the units arepresented by one of the curves indicated collectively by the referencenumeral 70 in the graph of FIGURE 7. The changes in the dissipationfactors of the corresponding capacitors are represented by the curvesindicated collectively by the reference numeral in the graph of FIGURE8. The graphs include only data obtained over 450 hours, but the unitsexhibited no further change in capacitance over the period tested whichwas in excess of 1000 hours. It will be noted that the averagecapacitance change is approximately the same at 0 volts DC as at 75volts DC, which indicates that no deterioration is caused by voltagestress. It will be noted in FIGURE 8 that an appreciable change in thedissipation factor occurred during the first 24 hours, but a definitetendency towards stabilization was subsequently exhibited. It isbelieved that this initial effect was caused primarily by smallquantities 'of moisture remaining in the encapsulation ambient whichsubsequently dissipated, or may have been caused by electrodedeterioration.

Another important aspect of the present invention is illustrated by thenovel capacitor construction indicated generally by the referencenumeral in FIGURE 9. A metal foil substrate 91 is placed in a reactionchamber such as the chamber 12, and suitable interlayers 92 and 94 aresimultaneously formed on the opposite surfaces of the foil using theprocess heretofore described. In the particular embodiment illustrated,the substrate 91 is molybdenum and the bonding interlayers aremolybdenum boride. Boron nitride films 96 and 98 are then simultaneous-1y deposited upon the metal boride interlayers 92 and 94, respectively,using the process heretofore described. Metallic film electrodes 100 and102 are then deposited on the boron nitride films by any suitabletechnique, but preferably by vapor condensation, to produce themultilayer capacitor 90. The metallic films are preferably of a metaldifferent from the substrate 91, such as gold, so that the two metalscan be selectively etched as will hereafter be described. Electricalcontact can be made with the electrodes individually and thereby producetwo separate capacitors, or the metal film electrodes 100 and 102 can beelectrically interconnected to provide a single capacitor having twicethe capacitance for a given area of substrate.

It will be appreciated that the capacitor construction 90 is paper thinand that making separate electrical contact with the electrodes createsa considerable problem. The electrical connections to the multilayercapacitor 90 can be made by either of two methods, the first of which isillustrated in FIGURE 11. The metal foil substrate 91 is out up intosmall pieces having the desired area. This leaves a sandwichconstruction having relatively square peripheral edges. As previouslymentioned, when the etching technique is to be employed, the metal filmelectrodes 100 and 102 will usually be a metal different from the metalfoil substrate 91. For example, the substrate 91 may be molybdenum andthe metal film electrodes 100 and 102 gold, so that the two metals willnot be attacked by the same etchants. Then the edge of the sandwich construction is dipped in a suitable etchant such as a solution of HCl-H NOhaving a high concentration of HCl. The relatively thin gold filmelectrodes 100 and 102 will be rapidly etched away relative to the rateat which the edge of the molybdenum substrate 91 will be attacked. Forexample, the molybdenum substrate 91 will be etched back to the point104 and the gold film electrodes will be etched back to the points 106and 108. The boron nitride films 96 and 98 may then be etched back tothe points 110 and 112, for example, by a solution of hot H PO asprevious ly described. A clip 114 can then be connected to the exposededge of the substrate 91 by conventional welding techniques.

.Another edge of the sandwich is then immersed in a suitable etchantwhich will selectively attack the molybdenum substrate 91 but will notaffect the boron nitride films 96 or '98 or the gold film electrodes 100and 102 so that the substrate 91 is removed back to point 116. Forexample, a solution of HCl-I-INO having a very low concentration of HClwill attack the molybdenum at a much greater rate than the gold and willnot attack the boron nitride. The electrodes 100 and 102 will tend tohold the otherwise fragile boron nitride films 96 and 98 intact and thefree edges of the films will tend to come together as illustrated inFIGURE 11 to cover the edge of the substrate. Then a clip 118 can becrimped over the edge of the sandwich and preferably welded to the metalfilms 100 and 102. The boron nitride covering the edge of the substrate91 insures that the clip 118 does not contact the substrate. Thecapacitor can then be encapsulated in a suitable insulating material.

An alternate technique for making contact with the electrodes of themultilayer capacitor construction 90 is illustrated in FIGURE 12. Whenusing this technique, an aperture indicated generally by the referencenumeral 120 is punched in the metal foil substrate 91 prior todeposition of the various layers. The metal boride interlayers 92 and94, and the boron nitride films 96 and 98 will come together and coverthe edge of the substrate 91 around the edge of the aperture 120. Thenwhen the metal film electrodes 100 and 102 are deposited, a film 122will also coat the edge of the aperture and provide electrical contactbetween the two electrodes 100 and 102. A suitable metal tack 124 maythen be inserted through the aperture 120 and welded by conventionaltechniques to the metal film electrode 100. If for some reason the metalfilm around the periphery of the aperture 120 does not providesufiicient electrical contact between the two electrodes 100 and 102,the other end 126 of the tack 124 can also be welded or otherwiseconnected to the metal film electrode 102. A suitable lead wire 128 canthen be welded to the tack 124.

Electrical connection is made to the substrate 91 by means of a secondtack 130. The metal film electrodes 100 and 102 are either etched awayin the areas 132 and 134 or the areas are masked prior to deposition ofthe electrodes to insure that the tack130 does not make contact witheither. Then the tack 130 is merely coated with suitable weld material,pressed through the metal foil substate in the boron nitride films 96and 98, and the device is heated to fuse the weld material and weld thetack 130 to the substrate 91. A suitable lead wire 136 can then bewelded to the tack 130.

Another multilayer capacitor constructed in accordance with the presentinvention is indicated generally by the reference numeral 140 in FIGURE10. The capacitor 140 is manufactured by placing a metal foil substrate141, such as molybdenum, in the reaction chamber and successivelydepositing a metal boride interlayer 142 and a boron nitride film 144 onthe substrate using the process steps heretofore described. A metal filmelectrode 146 is then deposited on the surface of the boron nitride film144 preferably by conventional vacuum-evaporation and condensationtechniques. The metal film electrode 146 should be chosen so that boronwill react with the metal to form a metal boride interlayer by the abovedescribed techniques. Molybdenum may be used for the electrode film.Then a second metal boride interlayer 148, a second boron nitride film150 and a second metal film electrode 152 are successively depositedupon the metal film electrode 146. This sequence of steps may berepeated any number of times to produce additional dielectric andelectrode films such as a third metal boride interlayer 154, a thirdboron nitride film 156 and a third metal film electrode 158. The secondmetal film electrode 152 may also be molybdenum, as illustrated, inwhich case the interlayer 154 would be molybdenum boride. However, itwill be appreciated that any one of the refractory metals heretoforespecified can be used in accordance with the broader aspects of theinvention.

An important aspect of the capacitor is that the boron nitridedielectrics 144, and 156 can each be made very thin yet can be made ofdifferent thicknesses so as to provide a variety of capacitances in thesame device. For example, the boron nitride film 144 might have theleast thickness, the boron nitride film 150 a greater thickness, and theboron nitride film 156 the greatest thickness. The thicknesses of theboron nitride films can be controlled with considerable accuracy aspreviously mentioned, and the total thickness of the films deposited onthe substrate might be less than one mil.

Electrical contact can be made with the electrodes of the device 140 inthe manner illustrated in the transverse cross section of FIGURE 13 byusing either masking or selective etching techniques. When using themasking technique, a relatively long strip of metal foil 141 may be usedas the substrate placed in the reaction chamber for deposition of thefirst interlayer 142, the first boron nitride film 144 and the firstmetal film electrode 146. Then the area is covered by a suitable maskextending along the entire length of the substrate 141. Then the secondinterlayer 148, the second boron nitride film 150 and the second metalfilm electrode 152 are deposited. The area 162 is then covered byadditional masking before the third interlayer 154, the third boronnitride film 156 and the third metal film electrode 158 are deposited.Then the masking can be removed and the elongated strip of foil 141 outtransversely into the desired number of capacitor sandwich units. Theconductors 164, 166, 168 and can then be welded to the substrate 141 andthe first, second, and third metal film electrodes 146, 152 and 158,respectively. The edges of the capacitor can be cleaned by suitableetching material to remove metals which have wiped over the cut edgesand would otherwise tend to short out the capacitor. The capacitor canthen be encapsulated in a suitable insulating material in a conventionalmanner, if desired.

The alternative process for attaching the leads to the variouselectrodes of the capacitor 140 involves the successive and selectiveetching of the various layers. When using such a technique, the metalfilm electrodes are preferably of a metal, such as molybdenum, that willnot be attacked by the H PO etchant for boron nitride. but which will beattacked by an etchant in which the boron nitride is stable, such as asolution of HCl-HNO The third metal film electrode 158 is first etchedaway by HCl-HNO to expose the third boron nitride film 156 in both areas160 and 162. During this step the third boron nitride layer 154 willprotect the second metal film electrode 152. Next the third boronnitride film 156 in the areas 160 and 162 is etched away by hot HPOpThen the third metal boride interlayer 154 is etched away in theareas 160 and 162 to expose the second metal film electrode 152 overboth areas. The second metal film elec trode 152 and the second boronnitride film 150 can then be successively etched away in the area 160 toexpose the first metal film electrode 146. The leads 164, 166, 168

and 170 can then be connected by some conventional 9 technique such aswelding and the capacitor encapsulated if desired by a suitableinsulating material.

Another important aspect of the invention is that a micro-circuitcomprised of a resistor and a capacitor connected in series can beconstructed in accordance with the present invention merely bycontrolling the thickness and properties of the metallic film electrode.For example, the electrode film 46 in FIGURE 3 could be a highresistance metal such as tantalum rather than alu-- minum. The thicknessof the metal film can be accurately controlled by controlling theparameters of the vapor phase deposition process so that a resistance ofthe desired value can be attained. The capacitor formed by the substrate40, the boron nitride film dielectric 44 and the electrode film 46 isthen connected in series with the resistor formed by the metal film.

From the above description, it will be evident to those skilled in theart that novel and highly useful electrical capacitors can bemanufactured. A capacitor not heretofore available has been describedwhich is comprised of a dense, continuous, pure and flexible dielectriclayer which is strongly adherent to a metal foil substrate. The boronnitride is a material which has not heretofore been available in thisform and in such thin films. Further, novel multilayer capacitors of asize, quality, and character heretofore unobtainable have beendescribed. The boron nitride dielectric films of the novel capacitors ofthis invention ranged in thickness from 0.2 to 5 In. microns inthickness. The boron nitride films are very uniform and are continuouslybonded to the substrate by the interlayer. The boron nitride filmsexhibited a dielectric strength of 200 volts DC per micron of thicknessand 5000 volts DC per mil thickness at room temperature. A calculatedcapacitance of 50,000 micromicrofarads per square inch for a 0.5 micronthick boron nitride film has been exhibited. The boron nitride films arestable in air up to 800 C. and can protect the substrate metal againstoxidation. The boron nitride film is stable in most acids and alkalisincluding aqua regia, HF-l-INO and NaOH. Therefore, the novel capacitorscan be used in high temperatures even in the presence of otherwisedeleterious ambients. However, we have discovered that hot H PO willattack the boron nitride film at a rapid rate such that novel selectiveetching techniques can be used in the construction of capacitors. Novelmultilayer capacitors have also been described which are highlyversatile and which permit very small devices to have very highcapacitance values.

Although preferred embodiments of the present invention have beendescribed in detail, it is to be understood that various changes,substitutions and alterations can be made therein without departing fromthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. An electrical capacitor comprising:

a metal electrode,

a boron nitride film chemically bonded to the metal electrode by a metalboride interlayer, and

a metal film electrode bonded to the boron nitride 2. An electricalcapacitor as defined in claim 1 Wherein:

the metal electrode is comprised of a molybdenum base :foil and themetal boride interlayer is essentially molybdenum boride.

3. An electrical capacitor as defined in claim 1 wherein:

the metal electrode is comprised of a tungsten base foil and the metalboride interlayer is essentially tungsten boride.

4. An electrical capacitor as defined in claim 1 wherethe metalelectrode is comprised of a niobium base foil and the metal borideinterlayer is essentially niobium boride.

5. An electrical capacitor as defined in claim 1 wherethe metalelectrode is comprised of a tantalum base foil and the metal borideinterlayer is essentially tantalum boride.

6. An electrical capacitor as defined in claim 1 wherethe metalelectrode is comprised of a ferrous alloy foil and the interlayer is aboride of at least one of the constituents of the alloy.

7. An electrical capacitor comprising:

a metal electrode,

a boron nitride film bonded to the metal electrode, the

boron nitride film being less than two mils in thickness, continuouslyadherent, flexible, and free from additives and binders, and

a metallic film electrode bonded to the boron nitride film.

8. An electrical capacitor comprising:

a metal foil electrode,

a boron nitride film chemically bonded to each side of the metal foil bya metal boride interlayer,

a metallic film electrode on each boron nitride film,

means for making electrical contact with the metal foil electrode, and

means for making electrical contact with the metallic film electrodes.

9. An electrical capacitor comprising:

a metal electrode,

a first boron nitride film chemically bonded to the surface of the metalelectrode by a metal boride interlayer,

a first metal film electrode vapor phase deposited on the first boronnitride film,

a second boron nitride film chemically bonded to the first metal filmelectrode by a metal boride interlayer,

a second metal film electrode vapor phase depositeo on the second boronnitride film, and

means for selectively making electrical contact with the metal electrodeand the first and second metal film electrodes.

10. An electrical capacitor comprising:

a metal electrode,

a boron nitride film chemically bonded to the metal electrode by aninterlayer comprising a compound of the metal of said metal electrode,and

a second metal electrode bonded to the boron nitride film.

References Cited UNITED STATES PATENTS 2,930,951 3/1960 Burger 317-258 X2,972,570 2/1961 Hass 3l7258 FOREIGN PATENTS 908,860 3/ 1960 GreatBritain.

OTHER REFERENCES Birks, J. B.: Modern Dielectric Material Heywood & Co.,1960, London, pp. 171-172 Qc585B5.

LEWIS H. MYERS, Primary Examiner. E, GOLDBERG, Assistant Examiner,

1. AN ELECTRICAL CAPACITOR COMPRISING: METAL ELECTRODE, A BORON NITRIDEFILM CHEMICALLY BONDED TO THE METAL ELECTRODE BY A METAL BORIDEINTERLAYER, AND A METAL FILM ELECTRODE BONDED TO THE BORON NITRIDE FILM.